Tropical Storm MIRIAM

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TROPICAL STORM MIRIAM DISCUSSION NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012
200 AM PDT SUN SEP 23 2012
MIRIAM HAS CONTINUED TO BECOME BETTER ORGANIZED SINCE THE PREVIOUS
ADVISORY...ESPECIALLY DURING THE PAST COUPLE OF HOURS. A SMALL CDO
FEATURE HAS DEVELOPED OVER THE CENTER AND CONVECTIVE BANDING HAS
ALSO BECOME MORE PRONOUNCED. SATELLITE INTENSITY ESTIMATES ARE A
CONSENSUS T3.0/45 KT FROM TAFB AND SAB...AND RECENT UW-CIMSS ADT
VALUES WERE T3.5/55 KT. BASED ON THESE DATA...THE INITIAL INTENSITY
IS INCREASED TO 45 KT. UPPER-LEVEL OUTFLOW IS GOOD IN ALL
QUADRANTS...AND CONTINUES TO EXPAND AND BECOME MORE SYMMETRICAL.
MICROWAVE SATELLITE IMAGERY OVER THE PAST 6 HOURS INDICATES THE
CENTER HAS DEVELOPED A LITTLE FARTHER TO THE NORTHEAST OF THE
PREVIOUS ADVISORY TRACK...AND THE INITIAL MOTION ESTIMATE IS NOW
310/07 KT. MIRIAM IS EXPECTED TO MOVE NORTHWESTWARD AROUND A
DEEP-LAYER SUBTROPICAL RIDGE FOR THE NEXT 72 HOURS. BY DAYS 4 AND
5...HOWEVER...THE TRACK FORECAST BECOMES A LITTLE MORE UNCERTAIN
DUE TO A LARGE SPREAD IN THE MODEL GUIDANCE. THE NEW 23/00Z ECMWF
MODEL MADE A MAJOR TRACK SHIFT TO THE EAST AND IS NOW THE RIGHTMOST
OF ALL THE MODEL GUIDANCE...WHICH IS MORE IN LINE WITH THE PREVIOUS
GFS MODEL RUNS. IN CONTRAST...THE HWRF AND GFDL MODELS HAVE
CONTINUED THEIR MORE WESTWARD MOTION IN THE LATTER PERIODS. GIVEN
THAT MIRIAM IS EXPECTED TO BECOME A FAIRLY ROBUST HURRICANE BY 72
HOURS...THE OFFICIAL FORECAST TRACK HAS BEEN SHIFTED TO THE RIGHT
OF THE PREVIOUS ADVISORY TRACK...AND FOLLOWS THE TREND OF THE GFS
AND ECMWF MODELS SINCE BOTH OF THOSE MODELS DEVELOP A DEEPER VORTEX
THAT IS LIFTED NORTHWARD ON DAYS 4 AND 5 BY A SHORTWAVE TROUGH
FORECAST TO DIG SOUTHWARD TO THE WEST OF BAJA CALIFORNIA.
ENVIRONMENTAL CONDITIONS ARE FORECAST TO BE QUITE FAVORABLE FOR
ADDITIONAL STRENGTHENING TO OCCUR FOR AT LEAST THE NEXT 48 HOURS.
IN FACT...WITH VERTICAL SHEAR VALUES ARE EXPECTED TO REMAIN BELOW
5 KT FOR THE NEXT 36 HOURS...AND IT WOULD NOT SURPRISE ME IF RAPID
INTENSIFICATION OCCURS AT SOME POINT DURING THAT TIME PERIOD. DUE
TO MIRIAM FORECAST TO BE FARTHER EAST AND OVER WARMER WATERS THAN
PREVIOUS EXPECTED ON DAYS 4 AND 5...THE INTENSITY WAS NUDGED UPWARD
SLIGHTLY AT THOSE LATTER TIME PERIODS...AND MIGHT NEED TO BE
INCREASED ON SUBSEQUENT ADVISORIES IF A FURTHER EASTWARD SHIFT IN
THE FORECAST IS WARRANTED. THE OFFICIAL INTENSITY FORECAST IS
SIMILAR TO BUT SLIGHTLY HIGHER THAN THE HWRF AND FSSE MODELS...
BOTH OF WHICH ARE ABOVE THE REMAINING NHC INTENSITY GUIDANCE.
FORECAST POSITIONS AND MAX WINDS
INIT 23/0900Z 15.3N 108.8W 45 KT 50 MPH
12H 23/1800Z 16.2N 110.1W 55 KT 65 MPH
24H 24/0600Z 17.2N 111.7W 65 KT 75 MPH
36H 24/1800Z 18.1N 113.2W 75 KT 85 MPH
48H 25/0600Z 18.7N 114.3W 85 KT 100 MPH
72H 26/0600Z 19.8N 115.7W 75 KT 85 MPH
96H 27/0600Z 20.8N 116.5W 65 KT 75 MPH
120H 28/0600Z 21.8N 117.0W 55 KT 65 MPH
$$
FORECASTER STEWART
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